Chocolate is an extremely popular and versatile food product which is characterized by a smooth, creamy consistency upon mastication. The desirable eating properties of chocolate are derived largely from the melting properties of cocoa butter which is typically present in chocolate at 25-35% by weight. The melting behavior of cocoa butter is the result of its unique triglyceride composition. However, this unique triglyceride composition, like the triglyceride composition of other natural fats, is relatively high in calories. Approximately 50% of the calories in chocolate products come from fat which is exclusively or predominantly cocoa butter. This means that persons who must restrict their intake of calories must either reduce the amount of chocolate products they consume, or in the extreme case, completely exclude such products from their diet.
Low calorie cocoa butter substitutes which have physical properties similar to cocoa butter have been developed. See, for example, U.S. Pat. Nos. 4,888,196; 5,023,106; and 5,066,510, Issued Dec. 19, 1989; Jun. 11, 1991 and Nov. 19, 1991, respectively, to Ehrman, Seiden, Weitzel and White (Ehrman et al). Such reduced calorie cocoa butter substitutes can conveniently be prepared, for example, by the acylation of monobehenin with caprylic and capric fatty acids or anhydrides. See U.S. Pat. No. 5,142,071 to Bernard W. Kluesener, Gordon Stipp and David Yang, Issued Aug. 25, 1992; and U.S. Pat. No. 5,142,072 to Gordon K. Stipp and Bernard W. Kluesener, Issued Aug. 25, 1992, especially Example 7, both of which are herein incorporated by reference.
Fatty acid anhydrides, including captic and caprylic fatty acid anhydrides, have been prepared by several known procedures such as dehydration of fatty acids using ketone, acetic anhydride, P205, phosgene or other dehydrating agents; thermal dehydration of fatty acids or soaps; acylation of fatty acids with fatty acyl halides; use of coupling agents such as dicyclohexylcarboiimide; acylation of fatty acids with enol esters and other methods as shown in R. Grimm, Fatty Acids, The American Chemists Society, ed. by E.H. Pryde, p. 218, 1979.
A preferred way to prepare food-grade fatty acid anhydrides is by the reaction of the higher fatty acids with a dehydrating agent, such as acetic anhydride. The thermal dehydration of fatty acids to form their anhydrides has generally been accomplished via a single-step, high temperature (e.g. &gt;175.degree. C.) disproportionation reaction. For example, acetic anhydride is reacted with fatty acids to yield a mixture of mixed and symmetrical fatty acid anhydrides, and, simultaneously, the mixed anhydrides are disproportionated to symmetrical anhydrides using stringent processing conditions (e.g., progressively lower vacuum and increasing temperature profile). The reaction is driven by the continual removal of acetic acid. See, for example, U.S. Pat. Nos. 2,981,744 issued to N. Ward et al on Apr. 25, 1961; 2,250,139 issued to C. Fuchs on Aug. 29, 1950; 2,246,599 issued to H. Oxely et al on Jun. 24, 1941; 2,026,985 issued to C Malm on Jan. 7, 1936; 2,221,026 issued to C. Malm et al on Nov. 12, 1940; 2,163,013 issued to H. Schultz on Jun. 20, 1939; and 2,411,567 issued to J. Fisher on Nov. 26, 1946.
The stringent processing conditions (e.g., high temperatures, low vacuum and long exposure times) typical of the disproportionation reaction were necessary to convert substantially all of the mixed anhydrides to symmetrical anhydrides. The presence of mixed C.sub.2 /C.sub.8 or C.sub.2 /C.sub.10 anhydrides results in the formation of acetin-containing triglycerides during the acylation of monobehenin. These types of fats exhibit an alpha phase, which is detrimental to formation of the desired beta phase in the reduced calorie cocoa butter substitute. (See Feuge et al, JAOCS, vol. 34 (1957), pages 239-244 and Feuge et al JAOCS, vol. 33 (1956), pages 367-371, both of which are herein incorporated by reference). Mixed anhydride levels below 0.5%, preferably below 0.2%, and most preferably below 0.1% are necessary for acceptable chocolate product performance.
Unfortunately, appreciable degradation occurs at these high temperatures (e.g. temperatures above 180.degree. C.) and long exposure times practiced in the prior art. This results in the formation orby-products such as color and odor bodies as well as di-fatty ketones. Typical di-fatty ketone levels of fatty acid anhydrides prepared by this type of disproportion reaction are in excess of 3000 ppm. Once formed, these di-fatty ketones and other by-products are difficult to remove and often render the resultant fatty anhydrides unsuitable for use in preparing cocoa butter substitutes or other food products.
It has now been found that fatty acid anhydrides which contain minimal by-products and which are suitable for food applications can be prepared from fatty acids and acetic anhydride by a fast, three-stage, low temperature process whereby mixed anhydrides are removed from the reaction mixture in a separate process step via thin-film short path evaporation rather than disproportionated.
Fatty acid anhydrides prepared according to this process will have purity levels of at least about 95%, di-fatty ketone levels of less than 300 ppm, good odor and minimal color formation compared to the starting fatty acids.. High purity cocoa butter substitutes can be prepared using stoichiometric amounts of these symmetrical acid anhydrides and long chain monoglycerides without the use of solvent or catalysts.